The recent reduction in size of hard disk drives has been bringing about an increased use thereof in handy music players and various kinds of storage devices. Under these circumstances, a further reduction in size and thickness, higher bearing accuracy (accuracy in the maintenance of smooth rotation) and reliability and a prolonged life have come to be required of fluid dynamic bearings used in spindle motors for driving hard disk drives.
The leakage of a lubricating fluid (so-called oil leakage) and its vaporization exist as factors hindering the reliability and prolonged life of fluid dynamic bearings. The leakage of a lubricating fluid is, among others, serious for mobile devices which are very likely to be subjected to vibration and impact from outside.
FIG. 7A is a top plan view showing the basic construction of a fluid dynamic being known in the related art, and FIG. 7B is a cross-sectional view, partly in section of FIG. 7A. The fluid dynamic bearing 900 shown in FIGS. 7A and 7B has a bearing member 904 supporting a shaft member (rotary shaft) 901 rotatably in a housing 905, an annular portion 901a of the shaft member formed in the shape of a flange as an integral part of the shaft member 901 to hold the shaft member 901 against detachment from the bearing member 904 and an annular member 903 on the bearing member which is fitted in the housing 905 to hold down the annular portion 901a of the shaft member at its edge.
According to the construction shown, a lubricating fluid continuously fills (1) a gap between the bottom of the shaft member 901 and the housing 905, (2) a gap between the inner periphery of the bearing member 904 and the outer periphery of the shaft member 901, (3) a gap 907 between the bottom of the annular portion 901 a of the shaft member and the top of the bearing member 904 and (4) a gap 906 forming a seal between the annular member 903 on the bearing member and the annular portion 901a of the bearing member to enable the shaft member 901 to rotate smoothly out of contact with the bearing member 904. The outer periphery of the annular portion 901a of the shaft member and the inner periphery of the annular member 903 on the bearing member are so shaped as to mesh with each other and the annular member 903 on the bearing member has a portion arranged to tie on the annular portion 901a of the shaft member along its edge to hold the shaft member 901 against detachment from the bearing member 904.
If the structure is subjected to impact or vibration, the shaft member 901 repeats its tendency to move to and away tom the bearing member 904 alternately. For example, the shaft member 901 tends to move away from the bearing member 904 at a first moment and move toward it at a second moment. At the first moment, the gap 907 tends to widen, while the gap 906 partly tends to narrow. At the second moment, the gap 907 tends to narrow, while the gap 906 partly tends to widen.
It is ideal that the lubricating fluid moves between the gaps 906 and 907 fast enough in response to any sudden change in size of the gaps to enable the seal to perform its function of preventing any leakage of the lubricating fluid. More specifically, it is ideal that the lubricating fluid moves from the gap 906 to the gap 907 at the first moment, while from the gap 907 to the gap 906 at the second moment, so that the movement of the lubricating fluid may prevent any unbalance occurring to its pressure and thereby any leakage thereof effectively.
However, as the fluid dynamic bearings make a further reduction in size and thickness, and as it is more likely to be subjected to a heavier level of vibration or impact, the gaps defining the seal change their dimensions so rapidly that any smooth movement of the lubricating fluid in the seal may be difficult.
More specifically, if the shaft member 901 tends to move up relative to the annular member 903 on the bearing member at a speed above a certain level, the lubricating fluid fails to move smoothly in a timely way from that part of the gap 906 having a rapid reduction in dimensions to the gap 907 having a rapid increase in dimensions. If such is the case, the gap 907 has a reduced pressure instantaneously and the gaseous components dissolved in the lubricating fluid form bubbles in the gap 907. As the formation of bubbles is a phenomenon corresponding to the sudden formation of expanding components in the lubricating fluid, the repeated exposure to impact or vibration causes the lubricating fluid to be forced out and leak through the opening 908. Even if no bubbles may be formed, an unbalance in the pressure of the lubricating fluid makes it likely to leak.
As the annular portion 901a of the shaft member and the annular member 903 on the bearing member meshing with each other, as shown in FIGS. 7A and 7B, require the lubricating fluid to move along a bent path and thereby disable it to move rapidly, the unbalance in pressure and the formation of bubbles are particularly likely to occur as stated above. Accordingly, the leakage of the lubricating fluid is likely to occur.
Because of its construction, the fluid dynamic bearing inevitably has a portion where the surface of the lubricating fluid is exposed as shown at 908 in FIGS. 7A and 7B, and where the vaporization of the lubricating fluid occurs. The vaporization of the lubricating fluid results in a gradual reduction in the amount of the lubricating fluid which the bearing device holds. The fluid dynamic bearing finally loses its function as a bearing when it has ceased to retain the necessary amount of the lubricating fluid for maintaining its function as a bearing.
As regards the art of preventing the leakage of a lubricating fluid from a fluid dynamic bearing the disclosure of JP-A-9-79272 is, for example, known. FIG. 8 is a sectional view outlining the fluid dynamic bearing described in JP-A-9-79272. According to its arrangement, the structure in which a shaft member 21 is supported by a bearing member 22 has a lubricating fluid reservoir 28 for storing an excess of lubricating fluid to prevent the leakage of the lubricating fluid through a seal 27. According to its arrangement, the lubricating fluid reservoir 28 has an opening area smaller than that of the seal 27, so that owing to its surface tension, the fluid may have a higher surface level in the lubricating fluid reservoir 28 than in the seal 27, so that any leakage of the lubricating fluid through the seal 27 may be prevented more effectively.
However, the structure shown in FIG. 8 does not consider any function of supplying the lubricating fluid from the lubricating fluid reservoir 28 to the seal 27, since the lubricating fluid reservoir 28 has an opening area smaller than that of the seal 27 to rely on the surface tension of the fluid for keeping a higher surface level in the lubricating fluid reservoir 28 than in the seal 27. Therefore, the lubricating fluid reservoir 29 cannot be expected to prevent effectively the formation of bubbles originating from the failure of the lubricating fluid to move rapidly in the seal as stated before.
As the lubricating fluid has a higher surface level in the lubricating fluid reservoir 28 than in the seal 27, the lubricating fluid partly remains unused in the lubricating fluid reservoir 28 in the event that its surface level in the seal 27 has dropped to a level below the end of the bearing member 22 due to e.g. the evaporation or leakage of the lubricating fluid. This means that the bearing is so constructed as not to permit tie complete use of the lubricating fluid in the lubricating fluid reservoir 28, which is not desirable for prolonging the bearing life to the maximum possible extent. Thus, the structure shown in FIG. 8 has not been satisfactory in the effect of preventing the leakage of the lubricating fluid, nor has it been satisfactory in the measure against the vaporization of the lubricating fluid.